For the purpose of alloying molybdenum to steel, molybdic trioxide is the common molybdic oxide used. The molybdic trioxide is generally added together with the scrap charge in electric arc-furnaces. Molybdic trioxide may be formed and packaged as powder in drums, powder in cans or as briquettes.
Molybdic trioxide is volatile at steelmaking temperatures. Standard handbooks give the melting point of molybdic trioxide as 782.degree..+-.5.degree. C. (1440.degree. F.) and state that it sublimes. When molybdenum trioxide is added to molten steel baths, high losses due to the formation of molybdic trioxide gas are encountered. When used as an addition to steel converters, the gas forms as a hot jet and is accompanied by the production of intense smoke which penetrates the steel works. The hot jet of smoke can damage equipment outside the converter and, unless special precautions are taken, damage the converter as well. The sudden formation of gas produces a sound similar to the detonation of a small bomb.
Because of the limitations presented by molybdic trioxide, ferromolybdenum, which is considerably more expensive, is normally used as the agent for adding molybdenum to a molten steel bath. There is great need for an agent which would operate with less pyrotechnics and which is less inexpensive than ferromolybdenum.
It is known to produce molybdenum trioxide commercially by roasting molybdenite (i.e., MoS.sub.2, the principal ore of molybdenum). Roasting is usually accomplished in a multi-hearth furnace of the Herreshoff type. U.S. Pat. No. 4,034,969, which is incorporated herein by reference, describes such a furnace and a means of controlling temperature therein which employs water jets as well as control of air flow to the various hearths. As pointed out in the patent, the use of increased air flow to control temperature on a particular hearth is not completely effective since air admitted to a hearth tends to flow upwards as well as across the hearth.
Increase in total air flow to the furnace results in dilution of the SO.sub.2 content of the exit gas which is undesirable for a number of reasons. For example, where SO.sub.2 is recovered in a sulfuric acid plant, this operation is more efficient when a rich gas is employed. Desirably, the SO.sub.2 content of the exit gas should be 2% or 3% or more. Increase in total gas flow raised many other costs in terms of equipment size, larger dust collection facilities, etc. It is accordingly desirable to operate the roaster with the lowest gas flow consistent with temperature control and completion of roasting.